Did you have to set the gateway for your other PCs.
Under default conditions, the router advertises the gateway and the PC will adopt the settings automatically.
If you have other wireless connections and you didn't manually set their gateway address and they work, then the other PC should do the same.
Are you using any form of encryption or mac address filtering on the router.
What service pack is installed on the disconnected PC?
Have you tried using a wired connection temporarily?
Have you tried running the network setup wizard?